Prepolymers presently available commercially for the casting of polyurethane elastomers are either of the polyester type or the polyether type; the latter, for example, are often based on polytetramethylene glycol (PTMG), polyoxypropylene glycol (PPG), or modifications thereof with polyoxyethylene.
Cast elastomers prepared from polyester prepolymers in general have better mechanical properties, especially at higher operating temperatures, and better hysteresis properties than elastomers prepared from polyether prepolymers. In addition, they are normally less expensive than polyurethane elastomers prepared from polyether prepolymers based on PTMG. On the other hand, polyether-based polyurethane elastomers have better hydrolytic stability and low temperature properties than elastomers based on polyester prepolymers.
While it might seem that polyurethanes having desirable properties might be obtained by using a mixture of both polyester and polyether components, and in fact this has been attempted to some extent as noted below in different types of systems, at present there are no commercially available polyester-polyether co-prepolymers. One reason for this is that it is accepted in the industry by both manufacturers of prepolymers and end-use fabricators, that polyester prepolymers and polyether prepolymers are generally incompatible and cannot be used in combination.
True prepolymers are made from reactants which have a total NCO/OH ratio on the order of, but no greater than, 2/1. The resultant prepolymers are thus isocyanate terminated without residual hydroxy terminal groups, e.g. ##STR1##
On the other hand, if the NCO/OH ratio of the reactants is greater than 2:1, what results is a mixture of free or unreacted diisocyanate with prepolymer, termed a quasi-prepolymer. However, such quasi-prepolymers are highly undesirable as products of commerce. The unreacted isocyanate renders the product unduly reactive and, thus, when used to make a cast polyurethane by mixing with a curing agent, the mixture reacts too quickly, e.g. before the mixture can flow and fill all parts of the mold. In addition, many free or unreacted isocyanates are both environmentally dangerous and relatively volatile; thus, great care must be exercised in ensuring that quasi-prepolymers are not subjected to heat.
As the ratio of NCO to OH approaches 1:1 from 1.5:1, the number of reactive NCO groups becomes progressively less, and at the NCO/OH ratio of 1:1, the product is no longer a prepolymer at all, but is a finished polyurethane polymer. At NCO/OH ratios less than 1, one obtains hydroxyl terminated prepolymers. These prepolymers, normally useful as components in coatings and sealant systems, are not conventionally used for casting and if so used, do not provide high performance elastomers. They are cured with different curing systems than are used for NCO terminated prepolymers, e.g. NCO terminated prepolymers may serve as curing agents for hydroxyl terminated prepolymers.
The 1961 U.S. Pat. to Mueller et al No. 2,998,403 broadly discloses reacting a diisocyanate with a linear hydroxy polyester and an hydroxy polyether, in which the ether oxygen atoms are separated by carbon chains of at least three carbon atoms. Various procedures are given for interreacting the three components and at column 2, lines 64-72, it is mentioned that the hydroxyl polyester may be first reacted with an excess of diisocyanate, followed by reaction with the hydroxyl polyether so as to provide an excess of hydroxyl groups, i.e. an NCO/OH ratio of les than one; subsequently, this hydroxyl terminated prepolymer is reacted with more diisocyanate to provide the final product. A true co-prepolymer of NCO/OH ratio greater than 1:1 and no greater than 2:1 is not provided, even transiently.
The Ellegast et al U.S. Pat. No. 3,274,160 discloses the preparation of a three component quasi-prepolymer in which excess polyisocyanate, at least 300% excess, is reacted with a polyhydric alcohol (e.g. trimethylpropane, 1,4-butenediol, glycerine, etc.) and an organic compound which is a polyester or polyether polyol of molecular weight greater than 800. The quasi-prepolymer so prepared is then in a later stage reacted with more polyhydric alcohol. No mixture of polyester and polyether polyols is suggested and, moreover, the NCO/OH ratio in the quasi-prepolymer is substantially in excess of 2:1.
The U.S. Pat. No. to Murphy 3,033,825 discloses casting polyurethane rubbers made from polyester prepolymers, e.g. using a glycol and adipic acid or isophthalic acid. Polyether glycols are also mentioned, but there is no disclosure of the preparation and isolation of isocyanate-terminated co-prepolymers containing both polyester and polyether glycols, nor is there any disclosure of the use of polyethers and polyesters together.
Murphy U.S. Pat. No. 3,098,658 discloses reacting a polyol, a polyether glycol and a diisocyanate to prepare finished polyurethane compositions. There is no disclosure of the preparation and isolation of isocyanate-terminated co-prepolymers containing both polyester and polyether glycols.
Kolycheck U.S. Pat. No. 3,493,634 shows the preparation of an injection moldable (thermoplastic) polyurethane formed by the reaction of a mixture of an hydroxyl-terminated polyester, an hydroxyl poly (alkylene oxide), an aliphatic glycol and an aryl diisocyanate. There is no mention of the preparation and isolation of a co-prepolymer containing a mixture of polyester and polyether polyols.
The above noted patents were found during a preliminary patentability search. Also found at that time were U.S. Pat. No. 3,528,948 to Reuter which shows the preparation of thermoplastic polyurethanes, similar to Kolycheck discussed above; and the Werner U.S. Pat. No. 3,980,606 which relates to the preparation of prepolymers from mixed polyether polyols. Other patents found during this search, of somewhat less interest, and also not disclosing or suggesting the preparation and isolation of co-prepolymers based on a mixture of polyester and polyether polyols, are: U.S. Pat. Nos. 2,729,618; 3,061,574; 3,094,495; 3,268,488; 3,370,996; and 3,963,681.